In Sudan, the southern army says a northern plane bombed its territory, injuring several people.
The bomb landed in the Northern Bahr Gazal state on Friday, said southern army spokesman Col Phillip Aguer.
He said the incident took place when the northern armed forces were pursuing Darfuri rebels near the border.
A northern army spokesman confirmed that troops had engaged the rebels, but said the fighting had taken place 23km (14 miles) north of the border.
The spokesman said he did not believe a bomb had landed in the south.
The United Nations confirmed a bombing had occurred and said it was sending a team to assess the situation.
New war fears
Tensions in Sudan have been rising in recent weeks, ahead of a referendum next year on possible independence for the south of the country - a consequence of peace deal in 2005 that ended a long civil war.
Earlier this week, ministers in charge of the armed forces from both the north and the south held a joint press conference to stress a new war was not an option.
But many analysts say a return to conflict around the time of the referendum or afterwards is a real possibility.
Southerners are likely to vote for independence, but most of Sudan's oil is in the south and there are real doubts about whether Khartoum will allow the region to split away.